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Dye Based Inks

Dye based inks have been the standard for production inkjet printing applications for over 40 years. These are water-based inks and common for imprinting and digital print applications.

Unlike the colorants in pigment inks, dyes are not insoluble particles that can be attached to air bubbles and flotated to the surface froth of a flotation deinking process stage. Dye inks therefore, are thought to be only flotation deinkable if the dye molecules can be fixed to components which are flotated, such as special paper coating components. In processes using a bleaching step, however, most dyes can de-colorized with reductive bleaching chemicals used in many current deinking mills. In all cases in which dyes cannot be flotated, however, dye inks lead to an undesirable coloration of the process waters and in the case of bleaching, to a higher bleaching chemical dosage requirement. In general, large amounts of water soluble colorants from any printing technology in a deinking plant can, therefore, contribute to significant difficulties.

In 2010, the DPDA conducted extensive research with CTP in Grenoble France, a leading research organization for paper and recycling technologies. The results were very positive and we conclude that some dye based inks printed on uncoated wood-free papers can be successfully de-colorized with widely used recovered paper bleaching processes in the deinking plants which utilize these processes. In fact the typical chemicals used to re-pulp the paper before deinking is sufficient to de-colorize some dye colorants. A link to this report can be found here.

Pigment Based Inks

Pigment based inkjet inks contain very small pigment colorant particles. Our research findings indicate that although the dispersed pigment size in the ink prior to printing is theoretically too small to attach to bubbles used in flotation deinking, in actuality the pigments and binders in the ink form a film of aggregated pigments during printing. The pulping step fragments this film into a size range that, in many cases, appears to be efficiently removed during standard flotation processing.

Many published results for inkjet prints show ‘good deinkability’ as defined for example by the ERPC scorecard and INGEDE Method 11, which means high levels in the incoming paper mix should be compatible with standard mill processes. DPDA research has found that many factors can improve the quality of deinked pulp produced from inkjet prints. Major factors include type or brand of paper used, pH of deinking process, deinking chemicals used, and, in the case of washing deinking for hygiene papers, the washing conditions. Several deinking research studies from independent industry researchers suggest that pigment based inkjet prints can be recycled in mixed paper grades up to 10% of the incoming paper mix without significant changes in the final deinked pulp quality.

Numerous studies are underway and links to current research from industry experts, suppliers, and DPDA members can be found here.